The main objective of the work proposed is to study the role of postreplication DNA repair processes during the initiation of hepatocarcinogenesis in vivo. Rats are treated with N-hydroxy-2-acetylaminofluorene at various doses and times relative to enhanced liver DNA synthesis caused by surgical partial hepatectomy. The doses and times will be sufficient to cause varying degrees of short-term inhibition of regenerative DNA synthesis followed by recovery and are being experimentally determined. Postreplication DNA repair processes occurring during the recovery of DNA synthesis from carcinogen treatment will be examined using several techniques. The presence or absence or error-prone DNA synthesis/repair processes will be determined in two ways. One way involves the in vitro determination of the fidelity with which crude DNA polymerase fractions isolated liver nuclei at appropriate times carryout DNA synthesis. The other way involves the use of cultured hypatocytes obtained from the livers of animals treated in vivo. Such hepatocytes will be infected with UV-irradiated Herpes virus to determine the effect of the prior in vivo treatment on hepatocyte mediated Herpes reativation and mutagenesis. Post replicaton repair will also be measured in vitro in cultured hepatocytes isolated from regenerating liver and thus still carrying out DNA synthesis in culture. Following carcinogen treatment, alkaline sucrose gradient centrifugation or alkaline elution will be used to detect discontinuities and their repair in newly synthesized DNA. The extent of occurrence of these various postreplication processes will be correlated with the amount of excision repair occuring between carcinogen treatment and induced DNA synthesis and with the ability of the treatment protocols to induce preneoplastic nodules in liver.